In vifro particles of Rous sarcoma virus were assembled with Gag protein expressed in E. Coli and E. coli RNA and examined in the STEM. They appeared very much like immature retroviruses and had masses of about 80 Md, corresponding to about 1200 Gag proteins per particle. Other constructions are being studied. In addition, several laboratories are becoming interested in measuring the masses of retroviruses since most data on them is about 20 years old. Some preliminary results for RSV have been obtained.